Q: I have this beautiful little creek (many of which are called "rivers" by native Coloradoans) running across our property just in front of the house. Do we need to do anything legally to form a pond of maybe about twenty-five acres (native Coloradoans call these things "reservoirs")?
A: You were very wise to ask first, because too many newcomers just assume and then do. Unfortunately, later they find much money was spent and lost because Colorado is not a riparian water state (able to put water on or bordering your ground to "reasonable" use), at least at this level.
There are so many issues, believe me, you don't want me to make a long laundry list of things to review and permits and Court orders that will be required.
Let's just look at a narrow band of water concerns and forget about other water, environmental, land use, zoning, and geologic issues. Just because water is present on the property, you have no more right to use it than I would have, except for certain domestic purposes.
The general use of the water is governed by the Colorado rule of prior appropriation (limited by the federal cases, Supreme Court decisions, statutes, and treaties - Indian or international).
In a nutshell, if someone downstream first "uses" the water, you can not infringe on that person's right to use water passing through your property, either on or below the ground surface. That includes slowing the flow, increasing evaporation or seepage, or delaying the time that water eventually reaches the "owner."
"Your water" is probably "owned" by someone or some entity whose predecessor may have started using it back in the late 1890's or early 1900's. In fact, if these people fail to use the water, there are any number of "owners" with empty cups legally waiting to catch any precious drops that comes on down the line.
Now you can file papers in Water Court in Greeley to get in line but do not hold your breath when it comes to filling your "pond." You might get lucky during the few weeks of spring floods and divert water as long as there is no "call on the river." But I would guess that if the house is right next to the "creek," then it is unlikely you get much of a strong, sustained flow during flood season.
Finally, I hate to tell you but you may want to go about forty miles upstream where you will find a "pond" near the headwaters of your "creek." This is an example of a "high reservoir" built to control the water passing through your property for delivery to the "owners."
And the reason why you have a nice steady stream flow is that a group has obtained a minimum stream flow agreement with several entities that "owned" the water. Thus, do not build your "pond," nor plant that orchard right by the stream, reduce your planned herd of cattle to a few animals (not the numbers you shared with me), and never alter the banks of the "wet lands" because that is part of the river.
Any of these activities (and many others) could be a "taking" of water from someone else whose interest was here long before you came and would prompt action against you by the State Engineer's office through its division office in Greeley.
But what can you do? Like many homeowners associations in the area, buy the water you will need to fill the "pond." Then you can tackle all of the other issues that I did not have column space to highlight (Hint - buy water that you can physically get to your place!)